When the U.S. tech bubble burst back in 2000, investors suddenly realized the market was full of solid, if boring, companies trading at low valuations. The same may be true in China now, and it could be an opportunity for foreign investors—who are about to get more access to the mainland stock market.

China's fastest-growing companies have suffered a share-price drop of almost 15% over the past three months. One reason is valuation:...